In more general terms, known frequency positioning apparatus includes various items that are common, as to their functions specified below, with apparatus of the present invention, namely:
an emitter for emitting an emission wave at a controlled emission frequency; and
emission position servo-control means including a positioning mixer that performs wave mixing to form a positioning beat signal and that controls said emitter in response to variations in the frequency at a defined spectrum distance position said emission frequency at a defined spectrum distance from a defined support frequency.
It will be understood that the word "wave" as used above is equally applicable to a wave that is propagating freely, to a wave that is guided, or to a signal that is propagating along a transmission line, and that the nature of said wave or signal may be electromagnetic, acoustic, etc. . . . .
Such apparatus is included in each terminal of a known communications network described in European patent document EP-A-0 381 102 (F.degree.16761). In such a terminal, a fraction of a locally generated optical emission wave is transmitted to one input of a positioning mixer whose other input receives other waves coming from other terminals, with the frequency of one of said other waves constituting said support frequency.
An object of the present invention is to improve relative positioning of a frequency.
Another object is to facilitate implementing a frequency-multiplexed communications network in which frequencies that are adjacent in a succession of emission frequencies are to be separated by a frequency increment, and in which said frequency increment must be large in order to enable messages to be transmitted at high data rates.